There are times when sending that a particular ISP will be busy or  be having issues - and our servers (at BBCS) will attempt for a certain period of time to deliver the message - and in the event that they cannot deliver in the time limit, it will abort the process and messages like this will pop up and it just means that the email server (ex. Verizon, AOL, Gmail, etc) wasn't accepting the messages at the time.

You'll want to try to resend to the addresses, and it's likely they'll send without issue.